Programming PHP

The pval/zval Data Type

Throughout the PHP source code, you will see references to both pval and zval . They are the same thing and can be used interchangeably. The pval/zval is the basic data container in PHP. All data that is passed between the extension API and the user-level script is passed in this container. You can dig into the header files further yourself, but in simple terms, this container is a union that can hold either a long, a double, a string including the string length, an array, or an object. The union looks like this:

typedef union _zvalue_value {
        long lval; 
        double dval;
        struct {
                char *val;
                int len;
        } str;
        HashTable *ht;
        zend_object obj;
} zvalue_value;

The main things to learn from this union are that all integers are stored as longs, all floating-point values are stored in double-precision, and every string has an associated string length value, which, if properly checked everywhere, makes strings in PHP binary-safe.^[9] Strings do not need to be null-terminated, but since most third-party libraries expect null-terminated strings it is a good idea to always null-terminate any string you create.

Along with this union, each container has a flag that holds the currently active type, whether it is a reference or not, and the reference count. So the actual pval /zval struct looks like this:

struct _zval_struct {
        zvalue_value value;
        zend_uchar type;
        zend_uchar is_ref;
        zend_ushort refcount;
};

Because this structure could change in future versions of PHP, be sure ...